The acquisition of a flow cytometer by the user group included in this proposal would greatly accelerate and facilitate studies on the characterization of particular cell types with regard to cell surface phenotype, DNA content and F-actin content. These studies are critically dependent on the detection and quantitation of cells stained with fluorescent antibodies reactive with cell surface antigens, fluorescent dyes reactive with DNA, and fluorescent dyes reactive with cytoskeletal F-actin. In particular, the instrument will be able to examine a mixed cell population and simultaneously determine the distribution of cell size, cell morphology and relative levels of three distinct fluorescent reagents. The laboratories involved in this proposal intend to examine a wide range of cell types: B and T lymphocytes from normal mice, autoimmune mice, and aged mice as well as normal human T lymphocytes and fibroblast cell lines. Overall, these studies should lead to a better understanding of factors that control the activation and proliferation of various cell types, with a particular emphasis on lymphocyte subsets and their interactions. These studies will have significant clinical application to: (1) the control autoimmune disease; (2) the facilitation of bone marrow transplants; (3) the activation of components of the immune system in response to pathogens; and (4) the ability to prolong the effects on aging on the immune system.